In the field of fuel burners, particularly gas fired equipment such as gas furnaces, it had been common practice to provide a continuously lit or "standing" pilot. The use of a "standing" pilot was very inexpensive and was reliable in that the pilot was lit and normally was of sufficient size and strength to light a main burner whenever the fuel supply to the main burner was open. Basically this same concept existed in the oil burner market in that a constant source of spark ignition normally would be provided with a supply of oil and air to the burner.
Since the advent of the shortage and the subsequent higher price for gas and oil as fuels, efforts have been devoted to eliminate any unnecessary consumption of these fuels. In the field of gas fired equipment, the "standing" pilot is being replaced by intermittently operated ignition sources that either directly light a pilot for subsequent use as a light-off means for the main burner, or directly light the main burner itself. In the area of spark ignition for oil burners, an attempt has been made to utilize interrupted or intermittent ignition of the oil burner with the ignition source turned off immediately after a flame has been established at the oil burner. In both of these types of systems, a flame sensing arrangement is used to detect the presence of the flame for keeping the fuel valve open to continue the supply of main fuel to the burner. It has been found that at the initial lightoff of either the pilot or of a main burner, whether it be gas or oil, that an unstable flame may result. Many of the electric control systems now being proposed are fast enough in response so that the absence of, or momentary blowing of a flame away from a sensing element, causes the main valve to immediately close. This causes unnecessary cycling of the main valve and numerous attempts for the ignition system to restart the system. In systems that use a pilot light and a main burner, once the pilot is established, the main burner immediately comes on. This can cause a sudden movement in the air around the pilot flame which either blows the pilot flame out or moves it away from a flame sensing device, such as a flame rod. This unstable condition has been met in most prior art installations by very carefully selecting the type of pilot, the selection and placement of the main burner, and the very careful placement of the flame sensing means. This has added unnecessary complexity and expense to the design of gas and oil fired burner equipment.